Airbag system having multiple collision determining circuits

ABSTRACT

An airbag system comprises an airbag, an inflator, a squib, a high-side switch, a low-side switch, a firing circuit for switching the two switches, a first motion detecting circuit including an acceleration sensor, a second motion detecting circuit including an acceleration sensor, a first collision determining circuit for outputting a firing signal to the firing circuit, and a second collision determining circuit for outputting a firing signal to the firing circuit. The first collision determining circuit comprises an analog comparator for comparing a signal from the first motion detecting circuit with a reference value.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by referenceJapanese Patent Application No. 2003-77348 filed on Mar. 20, 2003.

FIELD OF THE INVENTION

The present invention relates to an airbag system having simple orlow-cost collision determining circuits.

BACKGROUND OF THE INVENTION

Recent cars have airbags that inflate to protect drivers or passengersin emergency. However, accidental inflation of the airbag may damagedrivers or passengers. In order to prevent such erroneous inflation, theconventional airbag system includes multiple systems for determiningwhen a car collision occurs. The multiple systems includes at least twosystem, that is, a main system and a safing (fail-safe) system.

Some safing systems carry out mechanical processing from sensing toswitching of inflator firing circuits. In view of recent demands forlighter weight and smaller size, however, many safing systems carry outelectric processing as main systems do. FIGS. 9 and 10 show thecircuitry of such airbag systems S of cars. The main and safing systemsof both airbag systems S carry out electric processing.

In FIG. 9, the airbag system S includes acceleration sensors 71 and 72,a firing circuit 2 and an airbag 9. The firing circuit 2 includes ahigh-side switch 21, a low-side switch 22 and a squib 8. Determiningcircuits 61 and 62 consist of A/D converters 611 and 621, microcomputers612 and 622, and signal output circuits 613 and 623, respectively. Thesignals output from the acceleration sensors 71 and 72 are convertedinto digital signals by the A/D converters 611 and 621. Themicrocomputers 612 and 622 process the digital signals to determinewhether the car has collided or not. When the car collides, the outputcircuits 613 and 623 output firing signals to turn on the high-sideswitch 21 and low-side switch 22, respectively, in the firing circuit 2.This causes a firing current to flow through the squib 8, firing theinflator to inflate the airbag 9. The circuitry shown in FIG. 10 issimilar to FIG. 9.

In the airbag systems shown in FIG. 9, the collision determiningcircuits 61 and 62 of the main and sating systems are providedindependently of each other, that is, built into separate electroniccircuit substrates, so that the system S is more reliable. However, theindependent circuit substrates require a larger area, so that theelectronic control units of the system are larger in size, and they makeit more difficult to reduce costs.

In the airbag system S shown in FIG. 10, the collision determiningcircuits for the main and safing systems are built into one electroniccircuit substrate 61 a in common, so that it is easier for the system Sto be smaller in size and less costly. However, the common unit makesthe main and safing systems more liable to fail at the same time, sothat the airbag system S is less reliable.

SUMMARY OF THE INVENTION

In view of the foregoing circumstances, it is an object of the presentinvention to provide an airbag system having reliable collisiondetermining circuits that are smaller in size and less costly than thoseof the conventional airbag systems.

An airbag system according to the present invention for a car includesan electronic control unit having two collision determining circuitsthat are independent of each other so that the system can be reliable.One of the collision determining circuits is simplified so that theairbag system can be smaller in size and less costly.

The simplified collision determining circuit comprises a comparator anddoes not include an A/D converter, a microcomputer, etc. The comparatorcompares a signal from a motion detecting circuit with a reference valueto determine whether the car has collided or not.

This enables the electronic control unit for the airbag system to besmaller in size and less costly than the conventional systems. Thesimplified circuit does not reduce the system reliability because thetwo collision determining circuits are provided to be independent ofeach other. The comparator as one of the collision determining circuitshas lower degrees of freedom of tuning etc. However, the other collisiondetermining circuit is sufficient for required collision determinationfor the particular car, for example.

Consequently, the circuitry of the airbag system can be simpler, and itis easy to make the circuitry smaller in size and less costly. When thecomparator is combined with a firing circuit or another circuit, thewiring between them is needless or shortened, so that it is inhibitedfrom picking up noise and/or other disturbance. This makes the airbagsystem more reliable. Herein, “combining” means combining or integratingtwo or more circuits having different functions, and does notnecessarily mean integrating them into one IC. Of course, when theswitch driving means of the firing circuit is a driving IC, it is morepreferable that the first collision determining circuit be built intothe driving IC.

Alternatively, the first collision determining circuit, which isindependent of the second collision determining circuit, is combinedwith the firing circuit. This maintains the reliability of the airbagsystem and makes the electronic control unit smaller in size and lesscostly. The first collision determining circuit includes an A/Dconverter, a determination operating circuit and a signal outputtingcircuit. Consequently, the first collision determining circuit as wellcan carry out the tuning etc. of the particular car. This increases thedegree of design freedom.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a block diagram of an airbag system according to a firstembodiment of the present invention;

FIG. 2 is a detailed circuit diagarm of an airbag system according tothe first embodiment;

FIG. 3 is a detailed circuit diagram of an airbag system according to asecond embodiment of the present invention;

FIG. 4 is a detailed circuit diagram of an airbag system according to athird embodiment of the present invention;

FIG. 5 is a detailed circuit diagram of an airbag system according to afourth embodiment of the present invention;

FIG. 6 is a detailed circuit diagram of an airbag system according to afifth embodiment of the present invention;

FIG. 7 is a detailed circuit diagram of an airbag system according to asixth embodiment of the present invention;

FIG. 8 is a detailed circuit diagram of an airbag system according to aseventh embodiment of the present invention;

FIG. 9 is a detailed circuit diagram of a conventional airbag system;and

FIG. 10 is a detailed circuit diagram of another conventional airbagsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail with reference tovarious embodiments shown in the accompanying drawings. The same and/orsimilar components in the following embodiments are denoted with thesame reference numerals.

FIRST EMBODIMENT

Referring first to FIG. 1, an airbag system S for a car has a carbattery 1, a controlling circuit 6, a boosting circuit 4, a firingcircuit 2, a squib 93, an inflator 92, and airbag 91. The controllingcircuit 6 controls the firing circuit 2 and boosting circuit 4. Thebattery 1 supplies power to the boosting circuit 4, which generates highvoltage. The high voltage is applied to the firing circuit 2, whichsupplies firing current to the squib 93 when the car collides. The squib93 fires the inflator 92, which produces gas for inflating the airbag91. The airbag 91 inflates with the gas.

The airbag 91, inflator 92 and squib 93 may be built into a steeringwheel 9 of a car. The firing circuit 2, boosting circuit 4 andcontrolling circuit 6 are incorporated into an electronic control unitfor the airbag system S. The boosting circuit 4 etc. are well known.

As shown in FIG. 2, the firing circuit 2 consists essentially of ahigh-side (high-voltage side) switch 21 and a low-side (low-voltageside) switch 22 for switching the firing current, which is supplied tothe squib 93. The firing circuit 2 includes a switch driving circuit,which is not shown, for driving the switches 21 and 22.

The controlling circuit 6 includes a safing collision determiningcircuit 61 and a main collision determining circuit 62, which controlthe driving of the firing circuit 2. The controlling circuit 6 alsoincludes a first motion detecting circuit 71 and a second motiondetecting circuit 72, which comprise a first acceleration sensor (afirst motion sensor) and a second acceleration sensor (a second motionsensor), respectively.

The safing collision determining circuit 61 is an analog comparator towhich a reference voltage REF is applied from a reference voltagesource, which may be a voltage dividing resistor circuit. The firstmotion detecting circuit 71 outputs an analog signal, which is comparedwith the reference voltage by the comparator. This determining circuit61 outputs low or high voltage, according to which the switch drivingcircuit of the firing circuit 2 turns on the high-side switch 21. Thesafing collision determining circuit 61 compensates (fail-safes) themain collision determining circuit 62.

The main collision determining circuit 62 is independent of the safingcollision determining circuit 61 and consists of an A/D converter 621, amicrocomputer 622 and a signal output circuit 623. The second motiondetecting circuit 72 outputs an analog signal, which is converted into adigital signal by the A/D converter 621. The digital signal is input tothe microcomputer 622. On the basis of the digital signal, themicrocomputer 622 determines whether to inflate the airbag 91 or not.When the microcomputer 622 determines that the car has had such acollision that the airbag 91 should be inflated, the output circuit 623outputs a firing signal to the switch driving circuit of the firingcircuit 2, turning on the low-side switch 22.

For more advanced and accurate determination, the microcomputer 622 alsobases the determination on an external signal EXT in addition to thesignal from the second motion detecting circuit 72. The main collisiondetermining circuit 62 carries out collision determination according tothe program stored in the microcomputer 622. Accordingly, by changingthe program, it is possible to easily change the setting of the systemfrom car to car.

When the two collision determining circuits 61 and 62 cause bothswitches 21 and 22 of the firing circuit 2 to be turned on, a firingcurrent flows through the squib 93 of this circuit. The firing currentfires the inflator 92, inflating the airbag 91.

SECOND EMBODIMENT

In the first embodiment, the safing collision determining circuit 61 andmain collision determining circuit 62 are provided independently in oneelectronic control unit.

In the second embodiment shown in FIG. 3, however, the safing collisiondetermining circuit 61 is incorporated into the firing circuit 2.Because this determining circuit 61 is a simple circuit comprising ananalog comparator, it is easy to incorporate the circuit 61 into thefiring circuit 2. The incorporation greatly reduces the possibility thatthe communication path from the determining circuit 61 to the firingcircuit 2 will pick up noise and/or the like. This makes the airbagsystem S more reliable. When the switch driving circuit of the firingcircuit 2 and the safing collision determining circuit 61 areincorporated into one driving IC, the electronic control unit can besmaller in size and less costly.

THIRD EMBODIMENT

In the third embodiment, shown in FIG. 4, in contrast to the secondembodiment, the safing collision determining circuit 61 and first motiondetecting circuit 71 are provided in one unit. Because this determiningcircuit 61 is a simple circuit comprising a comparator, it is easy toincorporate the circuit 61 into the detecting circuit 71. Theincorporation greatly reduces the possibility that the communicationpath from the detecting circuit 71 to the determining circuit 61 willpick up noise and/or the like. This makes the airbag system S morereliable.

FOURTH EMBODIMENT

In the fourth embodiment shown in FIG. 5, the safing collisiondetermining circuit 61 and main collision determining circuit 62 areassociated with the low-side switch 22 and high-side switch 21,respectively.

When the firing circuit 2 or another circuit fails by short-circuiting,short-circuiting to the ground occurs more possibly thanshort-circuiting to the positive power supply. The safing collisiondetermining circuit 61 is designed to detect a collision impact weakerthan that which the main collision determining circuit 62 is designed todetect. Accordingly, the association of the determining circuits 61 and62 with the switches 22 and 21, respectively, as shown in FIG. 5prevents the airbag 91 from inflating accidentally when theshort-circuiting to the ground occurs.

FIFTH EMBODIMENT

In the first embodiment, the safing collision determining circuit 61 andmain collision determining circuit 62 are connected directly with thehigh-side switch 21 and low-side switch 22, respectively.

In the fifth embodiment shown in FIG. 6, however, the switch drivingcircuit of the firing circuit 2 includes AND circuits 23 and 24 in placeof the direct connection. In this case, when the outputs from the twodetermining circuits 61 and 62 are not high at the same time, neither ofthe switches 21 and 22 is turned on, so that the airbag 91 does notinflate.

The comparison between the first and fifth embodiments reveals that theairbag systems S operate equally as far as the circuits are normal. Inthe first embodiment, however, when the low-side switch 22 fails with ashort-circuiting to the ground, the inflation of the airbag 91 isdetermined from the outputs from the first motion detecting circuit 71and safing collision determining circuit 61, independently of theoutputs from the second motion detecting circuit 72 and main collisiondetermining circuit 62.

In the fifth embodiment, even in such a case, the outputs from the twodetecting circuits 71 and 72 and the two determining circuits 61 and 62are reflected in the determination of whether to turn on or off thehigh-side switch 21, which is involved directly with the inflation ofthe airbag 91. This makes the airbag systems S more reliable.

SIXTH EMBODIMENT

In the sixth embodiment shown in FIG. 7, the safing collisiondetermining circuit 61 consists of an A/D converter 611, a microcomputer612 as a determination operating circuit and a signal output circuit613, as is the case with the main collision determining circuit 62. Thetwo determining circuits 61 and 62 are independent of each other. Thesafing collision determining circuit 61 and firing circuit 2 areprovided in one unit. Consequently, the degree of tuning freedom in thesafing collision determining circuit 61 can be equivalent to the degreeof freedom of the main collision determining circuit 62.

The switch driving circuit of the firing circuit 2 and the A/D converter611, microcomputer 612 and signal output circuit 613 are incorporatedinto a driving IC unit. The incorporation inhibits communication pathsfrom picking up noise and/or the like. This makes the airbag system Smore reliable.

SEVENTH EMBODIMENT

In each of the foregoing embodiments, the motion detecting circuits 71and 72 comprise acceleration sensors, which output analog electricsignals (voltage) according to the car motion (acceleration) at alltimes to the comparator, etc.

Essentially, however, the required function of the motion detectingcircuits 71 and 72 is to detect whether the car has had such a collisionthat the airbag 91 should inflate. Accordingly, the detecting circuits71 and 72 might output signals to the comparator etc. only when such acollision has occurred. FIG. 8 schematically shows such a motiondetecting circuit 73, which consists of a mechanical switch 73 a, aresistor 73 b and a power supply 73 c. One terminal of the mechanicalswitch 73 a is connected in series with the resistor 73 b. The otherterminal of the mechanical switch 73 a is connected with the powersupply 73 c. A midpoint between the mechanical switch 73 a and resistor73 b may be connected with a safing collision determining circuit 61.When the car acceleration exceeds a set value, the mechanical switch 73a is turned on so that the terminal voltage of the resistor 73 b may beoutput to the determining circuit 61.

This motion detecting circuit 73 might replace one or each of the motiondetecting circuits 71 and 72. For example, the detecting circuit 73might replace only the detecting circuit 71, which is connected with thesafing collision determining circuit 61.

The present invention should not be limited to the disclosedembdodiments, but may be modified in many other ways without departingfrom the spirit of the invention. For instance, more than twoacceleration sensors, collision determining circuits and switches may beprovided for actuating one airbag.

1. An airbag system for a car comprising: airbag inflatable with gas; aninflator for producing gas for the airbag; a squib for firing theinflator; a firing circuit including a high-side switch and a low-sideswitch for allowing a firing current to flow or preventing the currentfrom flowing through the squib, on the high-voltage and low-voltagesides, respectively, of the squib; the firing circuit further includinga switch driving means for switching the two switches; a first motiondetecting circuit including a first motion sensor for outputting anelectric signal representing a motion of the car; a second motiondetecting circuit including a second motion sensor for outputting anelectric signal representing a motion of the car; a first collisiondetermining circuit; and a second collision determining circuitindependent of the first collision determining circuit; the first andthe second collision determining circuits being adapted to determine onthe basis of the electric signals from the first and the second motiondetecting circuits, respectively, whether the car has collided or not;the two collision determining circuits being further adapted to outputfiring signals to the switch driving means of the firing circuit whenthe car collides, wherein the airbag is inflated when the car collides,the first collision determining circuit comprising a comparator forcomparing the electric signal from the first motion detecting circuitwith a reference value.
 2. The airbag system according to claim 1,wherein the second collision determining circuit is a main collisiondetermining circuit for determining whether the car has collided or not,on the basis of the electric signal from the second motion detectingcircuit and an external signal received from a circuit other than thesecond motion detecting circuit and representing the motion of the car,and wherein the first collision determining circuit is a safingcollision determining circuit for compensating the main collisiondetermining circuit without receiving external signals from circuitsother than the first motion detecting circuit.
 3. The airbag systemaccording to claim 1, wherein the first collision determining circuit iscombined with the firing circuit or the first motion detecting circuit.4. The airbag system according to claim 3, wherein the switch drivingmeans of the firing circuit is a driving IC, and wherein the firstcollision determining circuit is built into the driving IC.
 5. An airbagsystem for a car comprising: an inflatable airbag; a firing circuitincluding a plurality of switches for inflating the airbag when actuatedconcurrently; and a plurality of accelecration sensors for producingoutput signals respectively corresponding to impacts applied thereto; aplurality of collision determining circuit means for actuating theswitches, respectively, in response to the output signals of thecorresponding accelecration sensors, wherein the plurality of collisiondetermining circuit means includes at least a digital circuit means andan analog circuit means, the digital circuit means including an A/Dconverter and a microcomputer, and the analog circuit means including ananalog voltage comparator.
 6. The airbag system according to claim 5,wherein the analog circuit means is connected to actuate one of theswitches in response to an impact lower than that in response to whichthe digital circuit means actuates another of the switches.
 7. Theairbag system according to claim 5, wherein the plurality of switchesare provided in a single unit, and wherein only the analog circuit meansis provided within the single unit of the plurality of switches.
 8. Theairbag system according to claim 5, wherein the analog circuit means andone of the plurality of acceleration sensors corresponding to the analogcircuit means are provided in a single unit separately from other of theplurality of acceleration sensors and the digital circuit means.
 9. Theairbag system according to claim 5, wherein the analog circuit means isconnected to actuate one of the plurality of switches, which isconnected to a ground, and wherein the digital circuit means isconnected to actuate another one of the plurality of switches, which isconnected to a power supply side.
 10. The airbag system according toclaim 9, wherein the analog circuit means is connected to actuate theone of the switches in response to an impact lower than that in responseto which the digital circuit means actuates the another one of theplurality of switches.
 11. The airbag system according to claim 5,further comprising: a plurality of AND gates connected between theplurality of switches and the plurality of collision determining circuitmeans, respectively, wherein each of the AND gates are connected to boththe analog circuit means and the digital circuit means.